U9附件转base64

private string GetAttachmentBase64Content(Attachment attachment)
{
    Storage storage = new Storage();
    CS.Common.FileDBService.FileInfo file = storage.GetFile(attachment.FileHandler);
    if (file == null)
    {
        return null;
    }
    using (Stream stream = storage.GetFileContent(attachment.FileHandler))
    {
        stream.Seek(0, SeekOrigin.Begin);
        byte[] data = new byte[stream.Length];
        stream.Read(data,0,data.Length);
        return Convert.ToBase64String(data);
    }
}